Packet, cell and frame-based transport protocols are increasingly being used in telecommunications networks. In particular, Asynchronous Transfer Mode (ATM) technology is a flexible form of transmission which allows various types of service traffic, such as voice, video or data, to be multiplexed together on to a common means of transmission, the traffic being carried in cells each having a payload and a header indicating the destination of the cell. ATM statistically multiplexes traffic and removes the rigid relationship between traffic and the time-domain which exists in other multiplexing protocols. ATM can accommodate both traffic whose information rate is constant during a call, and traffic whose information rate varies during a call. Operators are investing in ATM transmission equipment to accommodate users' increasingly diverse communications requirements.
In order to minimise costs, a network operator wants to use his transmission network most efficiently. One way of achieving efficiency is to make the best use of the bandwidth of the network.
One known way of reducing the amount of bandwidth which a telephone call uses on a transmission network is to suppress the periods of silence which occur during a telephone conversation. Silence suppression detects silence at an input to a transmission network, does not send data representing silence across the network, and at an output of the network inserts `comfort noise` in place of the removed silence. The bandwidth that is saved by not transmitting the silent periods of a call allows other traffic such as bursty data traffic to be transported across the network. This sharing of the network is particularly easy to achieve in a cell-based network.
A technique for saving bandwidth on a cell-based transmission network is used in Northern Telecom's PASSPORT.TM. switch. The PASSPORT.TM. switch looks for consecutive identical cells representing an idle channel condition. When enough identical cells are received, the switch stops transmitting cells onto the network. At the destination the PASSPORT.TM. switch continues to transmit the last cell that it received, thereby saving bandwidth on the network. In this scheme the data which is suppressed at the input to the network is not regenerated accurately at the output of the network.
The present invention seeks to provide an alternative method of saving bandwidth.